Various photographic image-forming systems are known wherein the formation of images is accomplished by means of curable organic components having an ethylenically unsaturated group. Due to the presence of an ethylenically unsaturated group, the organic components undergo an imagewise light-induced addition reaction, which typically comprises either polymerization or crosslinking, and curing to form corresponding images.
Another known photographic image-forming system employs a combination of a leuco dye, which generates a color upon oxidation or reaction with an acid, and a radical generator which functions as an oxidizing agent or an acid. Examples of this type of system are described in Phot. Sci. Eng., 598(1961) and U.S. Pat Nos. 3,042,515 and 3,615,568.
The compositions used in these systems generally employ coinitiators. One of the coinitiators is a photosensitizer, which is thought to capture photons in incident radiation. The other coinitiators is called an activator. An activator is not directly responsive to incident radiation, but molecules of the activator interact with adjacent photosensitizer molecules excited by the capture of photons, and then the activator emits free radicals. In the case where a compound having ethylenically unsaturated groups is present, the free radicals thus emitted induce an addition reaction. Where a leuco dye is present, the free radicals induce a reaction of the leuco dye, whereby a color is either generated or disappears on the dye.
Various combinations of a photosensitizer and an activator, which function as described above, are known. A particularly useful activator is an s-triazine compound having at least one trihalomethyl group (hereinafter referred to as "trihalomethyl-s-triazine") because it is advantageously employed as a photopolymerization initiator for compounds having an unsaturated double bond. The trihalomethyl-s-triazine compound is also advantageously used as a material in photo-printing processes utilizing oxidation of a leuco dye or other dyes, or the reaction of such dyes with an acid. Hence, various combinations of trihalomethyl-s-triazines with photosensitizers have been proposed.
For example, these photosensitizers include the 3-ketocoumarin compounds disclosed in JP-A-58-15503 (corresponding to U.S. Pat. No. 4,505,793), the thiopyrylium slats disclosed in JP-A-58-40302, the napthothiazole merocyanine compounds disclosed in JP-B-59-28328 and JP-B-60-53300, and the merocyanine compounds disclosed in JP-B-61-9621, JP-B-62-3842, JP-A-59-89303 (corresponding to U.S. Pat No. 4,481,276) and JP-A-60-60104. (The terms "JP-A" and 37 JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively). By use of these photosensitizers, the spectral range in which the trihalomethyl-s-triazine compounds exhibit spectral sensitivity has been extended to the visible range, and especially in the region of light having a wavelength of 550 nm or shorter, wherein some sensitizers may be usefully employed.
However, a photosensitizer useful in the range of light having a wavelength of 550 nm or longer has not yet been developed. In addition, for practical use in the range of light having a wavelength of 550 nm or shorter, a more sensitive photosensitizer has been desired.